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I'm aware of the coastline paradox and its application to rivers in that the measured length of a river depends on the resolution of the measurement.

Is there an application that calculates the centre of a river (a line equidistant from either bank at all points), then adds points along the line according to the resolution specified (for instance, every metre, 10 metres, 100 metres, etc.), then joins the dots and takes an overall measurement?

Will
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    What you look for is the medial axis - you could use PostGIS for that: https://postgis.net/docs/ST_ApproximateMedialAxis.html – Babel Jan 03 '21 at 12:38
  • Nice idea, just make sure you have sfcgal enabled for this function https://gis.stackexchange.com/questions/167687/how-to-enable-sfcgal-in-postgis – Timothy Dalton Jan 03 '21 at 13:59
  • Raster least cost analysis could also be an option by creating a raster, covering the river polygon, and with pixel values larger towards the riverbank than in the middle of the river, so th cost will be minimized travelling in the middle of the river. See here (in the lower part: variant) for details: https://gis.stackexchange.com/a/474196/88814 – Babel Jan 20 '24 at 14:59

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There is no single application that will do what you ask as you are underestimating the complexity of the problem. Medial Axis will indeed identify a centre line but then you need to prune the network/tree, deal with islands, tributary junctions, different river types such as anastomosing or braided systems and if your source polygon that represents banks contains other topographic features like bridges, culverts and all other manner of river spanning structures then you need to deal with the gaps they create.

You don't give any indication of where in the world you intend to do your processing or the scale of it; a small sub-catchment or the entire Amazon river basin?

I would suggest you investigate the nations hydrological datasets or may be one of the DTM derived datasets? Many centreline river networks exist and constructing your own is a bit of fools errand unless you are working in an un-mapped region?

Hornbydd
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    I just learned a new word: anastomose! – Stu Smith Jan 03 '21 at 20:47
  • Pruning the tree to a single line was something I had considered, but features spanning the river would throw a spanner in the works (no pun intended), which I hadn't considered.

    This question was more out of curiosity, because I don't have access to a GIS application which would provide the functionality needed. I've recently been studying relatively small rivers (on a global scale) in the UK, so measuring them wouldn't be a mammoth task in and of itself.

    I would be interested in accessing hydrological datasets if they've already done the work for me. How do I do that?

     – Will Jan 04 '21 at 09:25
  • If you are working on UK rivers then you can download for free the OS OpenRivers dataset which is a generalised version of MasterMap, be aware it may have topological issues. If you want the full resolution version this is OS Waterlayer which you can access through edina. – Hornbydd Jan 04 '21 at 10:04
  • Thanks. I'll keep that in mind. – Will Jan 14 '21 at 11:35
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Tresco made a GPS for inland vessels. It shows river distance, and much more, like lowest bridge, list of all the bridges ect. Application runs on windows and is called just Tresco as far as I know. Everyone refers to it as such. And every inland ship I've seen so far has it. Might be expensive for private use though.

Vince
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Krzysztof Kruszyński
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